This alloy is designed to provide a high creep strength material capable of sustained operation at very high temperature, high stress conditions. It has been developed to provide a directionally solidified (DS), columnar grain alloy which imparts component creep performance and environmental properties (oxidation and hot corrosion resistance) similar to that of the first generation single crystal alloys. The alloy is designed for turbine engine airfoil components, the design of which makes them very difficult to be cast, or at best, impractical to cast using single crystal technology. The size and shape of some components, particularly turbine vane segments and the long, low pressure (LP) turbine blades now being designed for advanced technology, high performance engines are not currently capable of being cast, using current single crystal technology. Therefore, it has become very important to develop a DS alloy having characteristics approaching those of the first generation single crystal alloys to allow for further turbine engine performance improvements.
The new, more powerful, large, aircraft turbofan engines have increased diameter, and thus, increased length of the turbine blades. The LP turbine blade length (.gtoreq.8") required of certain advanced design turbofan engines is such that they cannot readily be cast as a single crystal. Such blades encounter an unacceptable loss of thermal gradient during the single crystal casting operation and the resulting castings begin to exhibit serious grain defects, such as moderate angle boundary defects which are referred to as "slivers", clusters of equiaxed grains known as "freckles", and high angle spurious grains. These grain defects particularly reduce the mechanical fatigue properties of the LP turbine blades.
A difficulty which this invention addresses is that the LP turbine blades for the new generation of very high performance aircraft turbofan engines require an alloy with creep strength in a temperature/stress environment which heretofore has necessitated the use of a single crystal superalloy. However, at the same time, the performance requirements necessitate blade designs of a configuration which cannot be readily cast using the single crystal technique. When it became evident several years ago that advanced turbine engine designs appeared likely to encounter such a barrier for lack of a suitable alloy from which to cast the new LP turbine blades of long length (.gtoreq.8"), the search for a new alloy was initiated. The starting point was the high performance and proven, directional solidification, columnar grain nickel-base alloy CM 247 LC (U.S. Pat. No. 4,461,659) because it is used to successfully cast the long LP blades and exhibits microstructural stability under high temperature/stress conditions. However, this alloy did not have the strength and creep resistance necessary for use under the temperature/stress conditions created by some of the newest, advanced turbofan engine designs. CM 247 LC requires a tightly controlled chemistry such that even small variations from the narrow range of its acceptable composition can and do have marked and often unpredictable effects upon the alloy's performance characteristics. The greater the variation from the optimum chemistry the greater the adverse effect upon its performance characteristics. Therefore, the problem was how could its strength be increased to more clearly equal that of a single crystal alloy without having a serious deleterious effect upon its other desirable characteristics, such as reduction of its resistance to grain boundary cracking during the DS casting operation, and increasing the alloy's tendency to form plate-like M.sub.6 C and TCP phase formation, among others. A further complication in the CM 247 LC chemistry as a base from which to work was the use of zirconium (Zr) as a creep strengthener. This presented a serious problem because whatever alloy was to be developed could not afford to lose the creep strength characteristics Zr provides in the CM 247 LC alloy, but at the same time, the tendency of Zr to migrate to and concentrate at grain boundaries could lead to serious DS castability problems.
Another problem encountered in the attempts to successfully cast as a single crystal, parts such as turbine engine vane segments, is that of the residual stresses created by the difference in both the rate and the amount of thermal contraction between the metal of the casting and that of the ceramic mold into which the metal has been poured. These stresses occur as the casting solidifies and cools. This condition can create excessive residual casting stresses. This residual stress can result in recrystallization during high temperature solution treatment of DS vane segments in alloys such as CM 247 LC. DS vane segments exhibiting recrystallization must be rejected due to the formation of undesirable transverse grain boundaries in the thin, upper or lower trailing edges of the vane airfoils. These transverse grain boundaries can nucleate thermal fatigue cracks during engine service. This situation is complicated and substantially worsened when the design of the vane segment (see FIG. 9) has significant overhang at the top and bottom shroud, which tend to hang-up in the mold and increase the stresses within the airfoils resulting from solidification and cooling. At the present time, there is no known way of effectively relieving these stresses and achieving full gamma prime (.gamma.') solutioning without occurrence of recrystallization.
Another important facet of this problem is that of creep. The alloy of this invention is designed for use in very high performance engines. In application, the blades must be confined as closely as possible within their surrounding turbine casing to hold to a minimum gas leakage between the blade tips and the casing. Creep becomes a very important factor in limiting the degree to which the clearance between blade tip and the casing can be closed without causing excessive contact and the danger of turbine blade damage or failure. Additionally, modern turbine engine design can require the integrally shrouded vane segments support a bearing and hence the requirement for improved creep strength DS superalloys. When single crystal technology can be used, the problem of creep is materially reduced. One of the objectives of this invention is to approach the single crystal creep strength characteristic while using the columnar grain, directional solidification casting technique.
Because the application of the advanced vane segments and the low pressure blades require use of an alloy which could produce functional characteristics quite close to those of a single crystal alloy, it was necessary to add a strengthening element to partially compensate for the shift from single crystal to directional solidification casting techniques and to compensate for the necessity to eliminate high temperature solutioning processes to strengthen the alloy. For this purpose, rhenium (Re) was added. While Re increases strength, it also is prone to creating alloy phase instability. Re, under high temperature (&gt;1700.degree. F. (927.degree. C.)) stress conditions can initiate the formation of plate-like, acicular appearing, topologically close packed (TCP) phases which are rhenium and tungsten rich and which if of sufficient size and frequency may nucleate premature mechanical fatigue cracks. The tests conducted on the alloy of this invention are evidence that the alloy composition herein disclosed has been able to take advantage of rhenium strengthening characteristics without the adverse effects resulting from creating excessive, large size and extensive rhenium, tungsten rich TCP phases. This has been accomplished by utilizing a very subtle balance of chemistry.
One of the advantages of single crystal superalloys is the ability to omit such elements as B, C, Zr and Hf because their basic purpose in high strength superalloys is that of strengthening grain boundaries. Omitting these elements increases the incipient melting point of the alloy. This has the advantage of permitting full solutioning of the .gamma.'. This makes an important contribution to the creep strength of the alloy. Some advance design vane segment castings made from DS type alloys cannot be solution treated to this extent because there is the danger of recrystallization in the casting. Consequently, their thermal fatigue strength can be adversely affected. Thus, a different approach to obtaining strength improvement was necessary which is the purpose of adding Re.